The invention relates to an electrolyte used in particular for electroplating or for coloring anodized aluminum and its alloys and containing Sn.sup.II ions and additions which are able to stabilize the Sn.sup.II ions.
Sn.sup.II salts in the form of e.g. sulphates, fluorborates or chloride solutions are used in acidic electrolyte solutions both for electroplating aluminum and its alloys and for coloring these materials after anodizing.
It is known that the acidic sulphate bath is practically the only one which contains simple Sn.sup.II ions. In all other electrolytes the tin is at least partly bound up in a complex form.
One difficulty in electroplating or in coloring oxide layers in acidic solutions is that, during the process, the Sn.sup.II oxidizes to Sn.sup.IV. By adding certain substances to the electrolyte the oxidation can be more or less suppressed i.e. the Sn.sup.II can be more or less stabilized.
The effectiveness of the stabilizing agent is decisive for the quality of the precipitate and therefore of the coloring of the oxide. It is generally known that the presence of both valency forms, Sn.sup.II and Sn.sup.IV ions, substantially impairs the quality of the surface treatment due to inadequate stabilizing of the Sn.sup.II. It is therefore clear that one must endeavor to keep the tin in acidic solutions in the Sn.sup.II form.
The most frequently used stabilizers in acidic electrolytes are organic aromatic compounds--often sulphonic acids, which contain amino or phenol group e.g.:
phenolic sulphonic acid PA1 divalent phenol: resorcinol, catechol, hydroquinone PA1 amino-phenol PA1 .beta.-naphthol PA1 dimethylaniline.
All of these compounds have, to a greater or lesser degree, a good stabilizing effect on Sn.sup.II. They can not however completely prevent a pair of the Sn.sup.II oxidizing to Sn.sup.IV. Consequently, the quality of the coloring of oxide layers on aluminum and its alloys is adversely affected. The depth of the dark color tones and the uniformity of the color is often difficult to achieve when the stabilizing of the Sn.sup.II ions is insufficient. This is a significant disadvantage of the stabilizers used up to now. An ideal stabilizer would be one which effects complete stabilization of the Sn.sup.II ions. In fact it is difficult to produce colors such as dark bronze or black for coloring anodized material, if using the above mentioned additions. Also, the throwing power of the electrolyte is unsatisfactory. This results in noticeably lighter and darker coloring of the edge zone. With progressive coloring time--after About 10 minutes--excessive coloring occurs and a metallic deposit is obtained on the surface, which then produces problems when cleaning the colored surface, contamination of the sealing baths and corrosion problems.
A further disadvantage of the substances employed to date as stabilizers for Sn.sup.II is their relatively high toxicity. The most widely used substances, the phenols, are particularly harmful to the environment. If they ever find their way into natural waters in large quantities, then the results are known to be very unpleasant. They are also difficult to break down biologically. In most countries therefore the environmental laws concerning phenols in waste waters are particularly strict. In Switzerland for example the upper limit for phenols tolerated in flowing waters is 0.05 mg/l. The limit for amines is 0.1 mg/l.
It is therefore an object of the invention to find an acidic electrolyte containing Sn.sup.II for coloring anodized aluminum and its alloys or for electroplating purposes, which, firstly contains substances with a better stabilizing effect on Sn.sup.II than those known up to now, secondly better throwing power and thus better distribution of current and thirdly is not harmful to the environment, or at least not to such a degree as those used to date, and overall satisfies best the various, to some extent contradictory, parameters.